igneous rocks
DESCRIPTION
Nature of Igneous Rocks, Magma, Lava, Textures, Types classification,compositions,Bowen’s Reaction Series, characteristics of magma, Origin of Magmas, Evolution of Magma, Magma Differentiation,Partial Melting,Fractional Crystallization, Plate Tectonic Setting of Igneous RocksTRANSCRIPT
Igneous Rocks
Sawtooth Mtns. near Stanley, ID-example of a intrusive igneous rocks….not to mentiona very cool place to do fieldwork……
Lava flow in Hawaii[magma @ the Earth’ssurface]
Pyroclastic FlowMontserrat Volcano
What is a rock? [Yes, you really do care.]
The Nature of Igneous Rocks• Form from Magma [Greek=“paste”]
– Hot, partially molten mixture of solid liquid and gas
– Gases: H2O, CO2, etc.
– less dense than
solid rock
– solidify upon
cooling
“The beer I had for breakfast wasn’t badso I had one more for desert.”-Sunday Morning Coming Downby Kris Kristofferson
• Magma vs. Lava– Magma: molten rock beneath the surface
– Lava: molten rock that has reached the surface
– Magma: form intrusive igneous rocks
– Lava: form extrusive igneous rocks
• Composition varies widely– Oxygen plus major elements
– Generally a silica (SiO2) melt
– Silica and water content control viscosity
– Silica content used in classification
Mafic Magmas
• Silica content ~ 50%
• High Fe, Mg and Ca
• High temperature molten magma– 1000o to 1200oC
• Major minerals:– Olivine - Ca Plagioclase
– Pyroxene
Silicic Magma
• Silica content: 65-77%
• High Al, Na and K
• Lower temperature magmas– Less than 850oC
• Major minerals:– Feldspars - Micas
– Quartz
Magma Viscosity-resistance to flow
• Controlled by silica and water content, and temperature
• As magma cools-silica tetrahedron form links
• Linkages control
viscosity:•High Silica=high viscosity•Low Silica=low viscosity
Magma Viscosity• High Silica=high viscosity/Low Silica=low viscosity
• Cooler Temperatures=higher viscosity/Higher Temperatures=lower viscosity
• Water and volatiles break linkages=lower viscosity
Occurrence of Igneous Rocks
• Found globally
• Formed in discrete geologic settings– Convergent plate margins
– Divergent plate margins
– Mantle plumes
Igneous Textures• Texture - the size, shape and relationship of minerals in the
rock
• Cooling history of the magma or lava
• Crystal size increases as rate of cooling slows
• Texture - the size, shape, and arrangement of interlocking minerals
• Factors affecting crystal size
• Rate of coolingSlow rate = fewer but larger crystalsFast rate = many small crystalsVery fast rate forms glass
Types of igneous textures Aphanitic (fine-grained) texture
Rapid rate of cooling Microscopic crystals May contain vesicles (holes from gas bubbles)
Phaneritic (coarse-grained) texture Slow cooling Large, visible crystals Kinds of igneous textures
Porphyritic texture Glassy texture Pyroclastic texture Pegmatitic texture
Exceptionally coarse grained Form in late stages of crystallization of granitic magmas
Glassy Texture• Very rapid cooling - quenched
– Volcanic glass
– Conchoidal fracture
– Rock is called obsidian
• No apparent crystalsLava flow quenchingin ocean water.
Glassy texture in obsidian
Crystalline Textures
• Crystal growth requires time for ions to migrate - form minerals
• Slow rate of cooling=time for crystal growth
• Crystals grow until melt is quenched or completely solidified
Aphanitic Texture• Fine grained texture
• Few crystals visible in hand specimen
• Relatively rapid rate of cooling
Aphanitic texture in rhyolite
Phaneritic Texture
• Coarse grained texture
• Relatively slow rate of cooling
• Equigranular, interlocking crystals
• Slow cooling = crystallization at depth
Phaneritic texture in granite
Pegmatites from the Sawtooth Mtns.-unusually largecrystals
Porphyritic Texture
Minerals form at different temperatures Large crystals (phenocrysts) are embedded in a matrix
of smaller crystals (groundmass)
• Well formed crystals (phenocrysts)
• Fine grained matrix (groundmass)
• Complex cooling history– Initial stage of slow cooling
– Later stage of rapid cooling
Pyroclastic Texture
• Fragmental appearance produced by violent volcanic eruptions
• Explosive volcanic eruptions
• Appear porphyritic with visible crystals– Crystals show breakage or distortion
• Matrix dominated by glassy fragments– Hot fragments may “weld” together
Granitic versus basaltic compositions Granitic [felsic] composition
Light-colored silicates Termed felsic (feldspar and silica) in composition High silica (SiO2) content Major constituent of continental crust
Basaltic [mafic] composition Dark silicates and Ca-rich feldspar Termed mafic (magnesium and ferrum, for iron) in
composition Higher density than granitic rocks Comprise the ocean floor and many volcanic islands
Other compositional groups Intermediate (or andesitic) composition
Contain 25% or more dark silicate minerals Associated with explosive volcanic activity
Ultramafic composition Rare composition that is high in magnesium and iron Composed entirely of ferromagnesian silicates
Silica content as an indicator of composition Crustal rocks exhibit a considerable range
45% to 70%
Silica content influences magma behavior Granitic magmas = high silica content and
viscous Basaltic magmas = much lower silica content
and more fluid-like behavior
Igneous rocks form as molten rock cools and solidifies General characteristics of magma
Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava Rocks formed from lava = extrusive, or volcanic rocks Rocks formed from magma at depth = intrusive, or plutonic
rocks The nature of magma
Consists of three components: Liquid portion = melt Solids, if any, are silicate minerals Volatiles = dissolved gases in the melt, including water vapor (H2O),
carbon dioxide (CO2), and sulfur dioxide (SO2)
Classification of Igneous Rocks
– Texture• Aphanitic
• Phanaritic
– Composition• Silicic
• Intermediate
• Mafic
• Ultramafic
Combination of Texture and Compositionproduces rock name
Origin of MagmasGenerating magma from solid rock
• Role of heat Temperature increases in the upper crust (geothermal gradient) average between 20oC to
30oC per kilometer Rocks in the lower crust and upper mantle are near their melting points Additional heat may induce melting
• Role of pressure Increases in confining pressure increases a rock’s melting temperature When confining pressures drop, decompression melting occurs
• Role of volatiles Volatiles (primarily water) cause melting at lower temperatures Important factor where oceanic lithosphere descends into the mantle
• Solid rock is at equilibrium with its surrounding• Changes in the surroundings cause solid rock magma [melting]• Lowering P
Mantle convection moves deep mantle rocks upwards• Raising T
Hot mafic magma intrudes into the crust• Changing composition
Adding small amounts of water
How do rocks melt?
Heat: Geothermal Gradient
Temperature of mostSilicic magmas [600-800oC]-about the base of crust
Temperature of MaficMagmas [1100-1200oC]-upper mantle conditions
A single volcano may extrude lavas exhibiting very different compositions
Bowen’s reaction series Minerals crystallize in a systematic fashion
based on their melting points During crystallization, the composition of the
liquid portion of the magma continually changes
Processes responsible for changing a magma’s composition
Magmatic differentiation Separation of a melt from earlier formed crystals
Assimilation Changing a magma’s composition by incorporating
surrounding rock bodies into a magma
Magma mixing Two chemically distinct magmas may produce a
composition quite different from either original magma
Magma Differentiation
• Magmas, and the resulting igneous rocks, show a wide range of compositions
• Source Rock – variations cause major and minor
variations in the magma
• Magma Mixing
• Assimilation
• Partial Melting– Individual minerals in source rock melt at
different T
– Magma is enriched in many elements, especially SiO2
How do we know this?Norman Bowen [1887-1956]-Experimental Petrologist-developed Bowen’s Reaction Series
Bowen’s reaction series
• Fractional Crystallization– Individual minerals precipitate from
magma at different T
– Magma is enriched in many elements, especially SiO2
– Magma may migrate [buoyancy]
Bowen’s reaction series
Plate Tectonic Setting of Igneous Rocks• Divergent Plate Boundaries
– Partial melting of mantle produces basaltic magma
• Convergent Plate Boundaries
– Subduction produces partial melting of basalt, sediments, parts of mantle
– Andesitic and rhyolitic magma
– Ascending magma assimilates lower crustal material
• Mantle Plumes
– Partial melting of plumes of mantle material
– Basaltic magma is produced
– Rising magma produce
• Intraplate island chains
• Flood basalt [Columbia River Basalts]